Sprinkler heads of this type have been known. A typical example is described in the specification of U.S. Pat. No. 4,618,002.
According to this prior art arrangement, as FIG. 9 shows, in order to stop the outlet port 4 of the sprinkler head body 1, there are disposed, under the plug member 29, a pair of holder members, upper and lower, 31, 32, between which a partially cutaway ring-shaped locking member 11 is held in expanded condition, partially exposed outside, under the clamping force of a screw member 30, and a washer-like spring 33 for upwardly biasing the plug member 29 is interposed between the upper holder member 31 and the plug member 29.
The pair of holder members 31, 32 are fixed in position by putting the locking member 11 held in expanded condition between the holder members 31, 32 into engagement with an upper tapered surface 36 of an inwardly raised portion 35 formed on the lower inner peripheral edge of the frame 34.
In the event of a fire or the like, the heat-meltable member 22 interposed between the collar portion 30a of the screw member 30 and the lower holder member 32 becomes melted to reduce the holding ability of the holder members 31, 32 with respect to the locking member 11, so that the locking member 11 is contracted under its own restoring force. As a consequence, the engagement between the locking member 11 and the inwardly raised portion 35 is removed; the holder members 31, 32 and the plug member 29 fall off the sprinkler head body 1; and thus sprinkling of fire-fighting supply water can be performed properly.
However, this prior art arrangement is disadvantageous in that since the locking member 11 is held in locking position by means of the pair of holder members 31, 32 and the tapered surface 36 of the inwardly raised portion 35 of the frame 34, when an external impact is applied to the sprinkler head, the locking member 11 is likely to shift along the tapered surface 36 of the inwardly raised portion 35 and, therefore, may not be maintained in its locking position, with the result that the plug member 29 or the like may drop untimely to cause fire supply water sprinkling.
Another disadvantage is that the locking member 11 is likely to slip off during the process of sprinkler head assembling, too, and therefore that the prior art sprinkler head is not suitable for production on a so-called automatic assembly line and is thus obliged to be produced through manual assembly operation.
Further, according to this prior art arrangement, when the locking member 11 is relieved of its locking condition, the plug member 29 falls by gravity only. This poses a problem that the plug member 29 does not drop quickly or accurately, and in particular that when the outlet port 4 has been closed by the plug member 29 for long, the dropping of the plug member 29 is apt to be hampered by rust or the like produced on the interface between the plug member 29 and the outlet port 4, which may be taken as a fatal disadvantage.